Enhanced electrical properties in sub-10-nm WO3 nanoflakes prepared via a two-step sol-gel-exfoliation method

Nanoscale Research Letters, Sep 2014

The morphology and electrical properties of orthorhombic β-WO3 nanoflakes with thickness of ~7 to 9 nm were investigated at the nanoscale with a combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), current sensing force spectroscopy atomic force microscopy (CSFS-AFM, or PeakForce TUNA™), Fourier transform infra-red absorption spectroscopy (FTIR), linear sweep voltammetry (LSV) and Raman spectroscopy techniques. CSFS-AFM analysis established good correlation between the topography of the developed nanostructures and various features of WO3 nanoflakes synthesized via a two-step sol-gel-exfoliation method. It was determined that β-WO3 nanoflakes annealed at 550°C possess distinguished and exceptional thickness-dependent properties in comparison with the bulk, micro and nanostructured WO3 synthesized at alternative temperatures.

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Enhanced electrical properties in sub-10-nm WO3 nanoflakes prepared via a two-step sol-gel-exfoliation method

Serge Zhuiykov 0 Eugene Kats 0 0 Materials Science and Engineering Division , CSIRO, 37 Graham Road, Highett, VIC 3190, Australia The morphology and electrical properties of orthorhombic -WO3 nanoflakes with thickness of ~7 to 9 nm were investigated at the nanoscale with a combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), current sensing force spectroscopy atomic force microscopy (CSFS-AFM, or PeakForce TUNA), Fourier transform infra-red absorption spectroscopy (FTIR), linear sweep voltammetry (LSV) and Raman spectroscopy techniques. CSFS-AFM analysis established good correlation between the topography of the developed nanostructures and various features of WO3 nanoflakes synthesized via a two-step sol-gel-exfoliation method. It was determined that -WO3 nanoflakes annealed at 550C possess distinguished and exceptional thickness-dependent properties in comparison with the bulk, micro and nanostructured WO3 synthesized at alternative temperatures. - Background The layered transitional quasi-two-dimensional (Q2D) semiconductor oxides MO3 (M = Mo, W), have recently attracted significant interest because they demonstrate quantum confinement effects at the few-layer limit [1,2]. Among them, tungsten trioxide (WO3) is an n-type semiconductor in an indirect bandgap of 2.6 to 2.9 eV [3] with excellent electrochromic and gasochromic properties [4]. It has electron Hall mobility of ~12 cm2V1 s1 at room temperature and responsive to the blue end of the visible spectrum ( < 470 nm) [5]. Extrinsic n-doping is therefore not required for WO3 to exhibit significant conductivity. Similar to graphene, WO3 can be mechanically or chemically exfoliated to provide fundamental layers. However, unlike graphene, which does not have bandgap, Q2D WO3 has rather large bandgap, making Q2D WO3 nanoflakes more versatile as candidates for thin, flexible devices and potential applications in catalysis [6], optical switches [7] displays and smart windows [8], solar cells [9] optical recording devices [10] and various gas sensors [11]. It has become one of the most investigated functional semiconductor metal oxides impacting many research fields ranging from condensed-matter physics to solidstate chemistry [10]. However, despite great interest of the research and industrial communities to the bulk and microstructured WO3, nanoscaled Q2D WO3 with thickness less than ~10 nm has received relatively little attention so far compared to its microstructured counterparts and to Q2D transitional metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te). In addition, last year's reports on alternative transitional semiconductor oxide Q2D MoO3 have exhibited exceptional thickness-dependent properties and the substantial increased of the charge carriers mobility (up to 1,100 cm2 V1 s1) in Q2D MoO3 [2,12]. It was also recently proven for MoSe2 that the reduction of bandgap can be achieved through decreasing the thickness of Q2D nanoflakes down to monolayer [13]. Therefore, realization of WO3 in its Q2D form can further engineer the materials' electrical properties, as quantum confinement effects in 2D form will significantly influence charge transport, electronic band structure and electrochemical properties [3]. More importantly, nanostructuring of WO3 can enhance the performance of this functional Q2D material revealing unique properties that do not exist in its bulk form [2]. The development of Q2D materials is generally a twostep process, the synthesis of the layered bulk material followed by the exfoliation process [14]. Although there is a wide range of controlled methods of synthesis available to produce different morphologies of WO3 nanostructures, such as microwave-assisted hydrothermal [15], vapourphase deposition [16], sol-gel [17], electron-beam [18] and arc-discharge [19], synthesis of Q2D WO3 is a topic that is yet to be widely explored. For instance, in a recent report, it was demonstrated that one possible way of bandgap reduction in bulk WO3 is to increase its sintering temperature [20]. However, what is the most favourable sintering temperature for exfoliation Q2D WO3 nanoflakes remains largely unexplored. In this work, we present for the first time new distinguishing thickness-dependent electrical properties of Q2D -WO3 obtained for nanoflakes with thickness below ~10 nm developed via two-step sol-gel-exfoliation method. These properties were mapped without damaging the sample by carefully controlling the sample-tip force. This is performed by using current sensing force spectroscopy atomic force microscopy (CSFS-AFM), also known as PeakForce TUNA [21], which allowed simultaneous measurements of the topography and the current flowing between the tip and the sample from the real-time analysis of force-distance curves measured for a tip oscillating in the kilohertz regime, far below the resonance frequency of the cantilever [22]. This technique also provided direct control of the force ap (...truncated)


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Serge Zhuiykov, Eugene Kats. Enhanced electrical properties in sub-10-nm WO3 nanoflakes prepared via a two-step sol-gel-exfoliation method, Nanoscale Research Letters, 2014, pp. 401, Volume 9, Issue 1, DOI: 10.1186/1556-276X-9-401